Many of the industrially-significant attributes of different steels (strength, hardness etc.) depend in part on the microstructure of the particular steel, that is the type or types of crystals of which the steel is composed and the grain size of the crystals. In typical steel manufacturing, the steel undergoes processing in order to produce a desired microstructure. Such processing typically includes thermal processing (including controlling the cooling rate of the steel to promote the formation of particular crystal structures in the steel) and mechanical processing (including reducing the thickness of the steel by rolling the steel, so as to, for example, cause recrystallization for the purpose of reducing the grain size of the steel). The attributes of a steel can also be affected by the addition of precipitation strengthening substances, that is, alloying substances that dissolve when the steel is heated and then tend to precipitate in the boundaries between the grains of the steel when the steel cools. The precipitate particles thus created build up resistance to slip between steel grains, thereby increasing the strength of the steel, particularly the yield strength.
The known precipitation strengthening substances suitable for use in steel include, niobium (referred to at times herein as Nb), titanium (referred to at times herein as Ti) and vanadium (referred to at times herein as V). Niobium typically combines with carbon (referred to at times herein as C) and possibly nitrogen (referred to at times herein as N), and precipitates as Nb(C,N) and/or NbC. Titanium typically combines with carbon and precipitates as TiC. Vanadium typically combines with nitrogen or carbon and precipitates as VN or VC. Niobium, titanium and vanadium may be present in steel for purposes other than direct precipitation strengthening and will, during typical steel production, combine with other alloying substances in the steel, but the above compounds (Nb(C,N), NbC, TiC, VN and VC) are those that are considered to be associated with, and significant for, ultimate precipitation strengthening. Titanium will also form TiN with nitrogen, but this is not a useful precipitation strengthening compound, largely because TiN forms and precipitates at relatively high temperatures, resulting in larger-than-desired precipitate particles (discussed generally in what follows). Various other possible precipitation strengthening compounds are also known, including: Ti(C,N), V(C,N) and TiNb(C,N).
The extent to which the addition of such precipitating substances increases the strength of the steel depends in part on the ultimate size and volume fraction of the precipitate particles. It is well known that the strengthening effect of such precipitation increases as the volume fraction of the precipitate particles increases and the precipitate particle size decreases. For a given volume fraction of precipitates, a smaller particle size means a higher number density of precipitate particles, that is, a higher number of interactions between precipitate particles and steel grains, and thus higher strength. With Nb(C,N) and/or NbC precipitation strengthening in ferrite steel, for a given volume fraction, the increase in yield strength attributable to precipitation strengthening increases by about one order of magnitude when the precipitate particle size is reduced from about 100 nm to about 3 nm.
For a given precipitating substance, precipitate particle size is primarily dependent on the temperature at which the particles form. Generally, the lower the temperature at which the precipitate particles form, the smaller the particle size. The volume fraction of the precipitate particles depends in part on the rate at which the precipitating substance diffuses within the solid metal. Generally the rate of diffusion is a function of temperature; a higher temperature resulting in a higher diffusion rate and thus a higher volume fraction of precipitate particles.
For some metals and some precipitating substances, the diffusion rate of the precipitating substance is sufficiently high at relatively low temperatures, (for example, room temperature) that significant precipitation strengthening occurs at these relatively low temperatures. Precipitation strengthening that occurs over time at room temperature, referred to as aging, generally produces relatively fine precipitate particles. For steel, the diffusion rate of the known precipitating substances is too low at room temperature to produce an appreciable volume fraction of precipitate particles, which means that aging does not result in significant precipitation strengthening. For example, although Nb(C,N) and/or NbC precipitation is thermodynamically possible in ferrite steel at relatively low temperatures, such as below about 500° C., because of the sluggish precipitation kinetics at these temperatures, only a minimal Nb(C,N) and/or NbC precipitation strengthening effect has been observed at these temperatures under industrial conditions.
It is known to reheat metals containing precipitating substances off-line to increase the rate of diffusion of the precipitating substances and thus increase the volume fraction of the precipitate. However, off-line heat treatment is generally not an effective way to enhance precipitation strengthening in steel. For steel and the precipitating substances known to be appropriate for steel, re-heating the steel to a temperature sufficiently high to increase the diffusion rate of the precipitating substance so as to increase the volume fraction of the precipitate particles within a commercially-reasonable period of heating time, generally results in a larger-than-desirable precipitate particle size. As well, off-line heat treatment of steel is costly and typically, and significantly, results in a loss of desirable microstructure characteristics of the steel. Therefore, off-line heat treatment is typically not the best technique for enhancing precipitation strengthening in steel.
What is needed is a process that increases the volume fraction of fine precipitates in steel so as to result in enhanced precipitation strengthening.